Collaborative Research: Cascadia Tilt

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EAR-0310335BilhamCOLLABORATIVE RESEARCH: CASCADIA TILT Great subduction earthquakes occur beneath the Pacific Northwest every few hundred years, and damaging but smaller earthquakes every few years. One surprising discovery (from GPS geodesy) is that every 13-14 months, an incremental shift of roughly 10cm occurs between the Juan de Fuca plate and North America some 20 km beneath the surface. This slow shift has a duration of 1-3 weeks, generates a low sub-sonic rumble (2s-5Hz), and most importantly, stresses the future rupture zone incrementally towards the next great earthquake. That is, a future slow slip episode may well be the final straw that breaks the camel's back - a possible Richter M=9 earthquake damaging both Seattle and Vancouver, and generating tsunami damage as far as Hawaii and Japan. In an attempt to learn more about these slow slip events, ten 1-km-long water pipes (tiltmeters) are being installed in Washington state each terminated by sensors that can detect 1 micron of relative water height change (a tilt equivalent of San Francisco rising the thickness of a sheet of paper relative to New York). The tiltmeters will monitor the next four slow episodes to ascertain precisely how and where slip occurs, and the relationship between these slow slip events and moderately damaging shallow earthquakes that appear to follow them. The data will be available in near real time in a web accessible format to engender public awareness. Graduate students at Central Washington University and the University of Colorado at Boulder will be involved in the not inconsiderable processing of the data. The project includes several aspects of public outreach, both to make the public aware of the details of the scientific method, and to ensure that the public, engineers, and urban planners in the region know the seismic hazards of the Pacific Northwest. Each slow event will be an opportunity to deliver news-breaking data, accompanied by a tailored message to the general public about the measurements and their significance to seismic hazards. At 14 months, the time interval between alerts is almost perfectly designed to remind the public that seismic safety is an ongoing issue, without overexposing them to false alarms. The end of two of the last several slow events has been marked by shallow, felt earthquakes, one of them (the Nisqually earthquake 28 Feb 2001), resulting in $250M of damage.